For the preparation of a latex product such as medical use gloves made of rubber, natural rubber latex has been used for long time as the starting material. The medical use gloves made of natural rubber latex has excellent elasticity and has been known to be a barrier to obstruct the moving of a pathogen contained in blood.
In a preparation method of a natural rubber gloves, it is generally performed to add sulfur as a vulcanizing agent or a vulcanization accelerator. Specifically, a mold or a former having shape of hand is dipped into natural rubber mixture to which a vulcanizing agent and an accelerator for one time or several times and a layer of desired thickness is laminated on the surface of the mold or the former. A rubber gloves of desired thickness is dried and crosslinked under heated temperature condition.
Crosslinking is a fundamental process to provide a natural rubber gloves with high elasticity. The natural rubber gloves prepared as above has excellent barrier ability, mechanical property and physical property.
Natural rubber latex contains less than 5% of non rubber components consisting of protein, lipid or other miner constituents. Along with increase of use of natural rubber latex gloves in hospital, users of I type hypersensitivity is increased. Cause of I type hypersensitivity caused by direct contact of a user to a natural rubber latex gloves is proved to be caused by latex protein existing in natural rubber which can be extractive. Hypersensitivity manifests immediately causes within 2 hours after contact with a gloves. This is caused by IgE (antibody in circulating blood) and allergic reaction is caused by IgE. The surface of skin displays uredo (hives) symptom which exceeds contacted point with latex.
Allergic symptom caused in whole body displays itching of eyes, turgid of lips and tongue, breathlessness, ache of abdomen, siccasia, extreme hypertonic state and unusually shock state by hypersensitivity.
And, the person who is progressing hypersensitivity symptom caused by natural rubber latex protein will be advised not to contact further with natural rubber latex and products made of natural rubber latex.
Since said protein is not contained in nitrile latex obtained by synthesis, carboxylated nitrile latex, synthesized latex such as poly chloroprene latex or polybutadiene latex, these latex does not cause said allergy. To the person who is progressed protein allergy, use of a gloves made of synthesized latex which uses nitrile latex, poly chloroprene latex or polybutadiene latex is recommended.
When a gloves made of these synthetic rubber latex is compared with a gloves made of natural rubber latex, from the viewpoint of physical property, the gloves of synthetic rubber latex is same or better than the gloves made of natural rubber latex, however, from the viewpoint of barrier property the former is inferior to the latter.
As the method for preparation of a medical use gloves from synthetic rubber, the same method as to natural rubber is used.
By same process to prepare a gloves using natural rubber, the thin film gloves of desired thickness can be obtained by sticking a composition of synthetic rubber elastomer on a mold or a former having shape of hand, then drying and crosslinking the composition. A gloves made of synthetic elastomer obtained as above can possesses desired mechanical property and physical property. Many gloves using synthetic rubber are developed and sold in the market. And as a starting material to prepare synthetic rubber gloves, carboxylized nitrile rubber is most commonly used.
In the method to prepare a gloves from synthetic rubber latex, sulfur is used as a crosslinking agent and sulfur containing compound is used as a crosslinking accelerator. Specifically, dithiocarbamate, tetramethyltiuram-disulfide (TMTD) or mercaptobenzothiazole (MBT) can be mentioned. These sulfur containing compounds can accelerate vulcanization. In a case of not using a vulcanization accelerator, vulcanization reaction is carried out by only use of sulfur. In this case, several hours treatment at high temperature condition of 140° C. is necessary, and the reaction progresses slowly.
At the preparation process of a rubber gloves, a vulcanization accelerator is used and consequently problems regarding health are caused. Concretely, these vulcanization accelerators make manifest contact determatitis delayed IVtype hypersensitivity which is allergic symptom. Delayed IVtype hypersensitivity causes after 24-72 hours from contact with a gloves. Generally, the symptom can be observed on the surface of hand or arm and causes dotted eruption, flare of skin and sometimes chaps of skin or bulla.    A gloves of nitrile rubber latex can be used instead of a gloves of natural rubber latex. In nitrile rubber latex, excess amount of vulcanization accelerator exists as residue. By stopping the use of a gloves of natural rubber latex, I type hypersensitivity can be avoided, however, delayed IVtype hypersensitivity by use of a gloves of nitrile rubber latex will be caused.
Accordingly, accomplishing of a method for preparation of a synthetic rubber gloves by excluding use of a sulfur crosslinking agent and a vulcanization accelerator becomes a pressing need. In a case to crosslink carboxylic nitrile rubber latex by not using sulfur crosslinking agent and a vulcanization accelerator of sulfur containing compound, it becomes necessary to crosslink a latex by a crosslinking agent not containing sulfur. As a non sulfur method, a crosslinking method by ionic bond using bivalent or trivalent metal zinc is now investigated. When this method is compared with sulfur method, formation of bonds is considered as to be same, however, from the viewpoint of physical property such as intensity and elongation, desired effect cannot be obtained and this is recognized as a problem. In the present invention use of bivalent metal salt such as zinc oxide is the essential point.
U.S. Pat. No. 5,014,362 (Patent Document 1) discloses a crosslinking method of carboxylic nitrile rubber using zinc oxide and sulfur. Typical carboxylic nitrile rubber is formed by segment consisting of acrylonitrile, butadiene and organic acid formed by various mixing ratio. By use of sulfur and a vulcanization accelerator, it becomes possible to form crosslinking by covalent bond in sub segment of butadiene. Further, in a part of carboxylated acrylonitrile (organic acid) ionic bond can be formed by using metal oxide such as zinc oxide or other metal salts. Ionic crosslinking is carried out by zinc ion and use crosslinking by sulfur too. When compared with a film by ionic crosslinking using zinc ion, physical property such as tension, braking intensity and abrasion resistance can be improved, however, problem caused by use of sulfur is not dissolved.
As mentioned above, in a case when mechanism of crosslinking is simply depending on ionic bond, rubber product becomes not reliable one in resistance to oil and chemicals.
In carboxylic nitrile rubber product such as gloves, to obtain effective crosslinking by combining covalent bond by sulfur and vulcanization accelerator with ionic crosslinking treatment by metal oxide such as zinc oxide and metal salt is an ordinary common sense.
Crosslinking method using said vulcanization accelerator results to cause new problem about health referring delayed IVtype hypersensitivity.
Since polymerization reaction is accelerated by using organized oxide, it becomes well known that intensity of rubber can be improved by adding organic zinc dimethacrylate and/or alkaline zinc methacrylate to rubber.
Polybuthadiene and methacrylic acid are mixed, then zinc oxide is added and a composition which is excellent in abrasion resistance can be obtained (Patent Document 2 JP53-125139 publication, Patent Document 3 JP52-121653 publication). By adding non polymerizing carboxylic acid to mixture of diene rubber, methacrylic acid, zinc oxide and organic peroxide, synthetic polymer having better tensile strength than natural latex (Patent Document 4 JP53-85842 publication). NBR crossliking becomes possible by using methacrylic acid, zinc oxide and peroxide, even if ionic bond is not existing.
Patent Document 5 (JPH08-19264 publication), discloses vulcanized rubber composition prepared by blending (b) 10-60 weight % of zinc compound whose content of coarser particles over than 20 μm is less than 5%, (c) 20-60 weight % of methacrylic acid and (d) 0.2-10 weight % organic peroxide to (a) 100 weight % of ethylene unsaturated nitrile—copolymerized conjugate diene rubber. Vulcanized rubber product which is excellent in intensity property can be obtained, and is useful for preparation of hoses, rolls and rubber vibration insulator, however, thin sheet such as gloves is not objected.
Soft nitrile rubber product which is rubber composition of high rigidity obtained by crosslinking under presence of sulfur hardening accelerator or non oxidated zinc (not containing bivalent zinc), having good tension and resistance against drug and characterized more soft than conventional products (Patent Document 6: Japan Patent 3517246 publication). In this document, tetramethyltiuramdisulfide combined with mercaptobenzothiazol (MBT) is used as a vulcanization accelerator. In soft nitrile rubber which is a reacted product consequently contains rubber.
Carboxylic nitrile rubber which is a copolymer of acrylonitrile, butadiene and unsaturated carboxylic acid can form ionic bond under presence of zinc and carboxyl group. However, it is difficult to form covalent bond by compound containing zinc, accordingly, the point to make up the defect by crosslinking with zinc using small amount of sulfur becomes very important point. Concretely, in JP2002-527632 publication (Patent Document 7) a gloves prepared by crosslink carboxylic nitrile rubber which is copolymer of acrylonitrile, butadiene and unsaturated carboxylic acid with 1-3 phr sulfur and polyvalent metal oxide.
In U.S. Pat. No. 6,673,871 publication (Patent Document 8), regarding elastomeric product such as gloves, the method to use metal oxide such as zinc oxide as a crosslinking agent by not using crosslinking agent or a vulcanizing agent containing sulfur is disclosed. In corresponding Japanese Patent JP2004-526063 publication (Patent Document 8), the method to crosslink and harden synthesized polymer at the temperature less than 85° C., said synthetic polymer is crosslinked by not using an accelerator and using crosslinking agent basically composed of metal oxide and a crosslinking agent which carries out sulfur substitution. Concretely, copolymer solution, which is on the market by the trademark of BARRIERPRO BP2000 from Reichhoold Chemicals Inc., is crosslinked under presence of water of specific concentration and zinc oxide which is a crosslinking agent. Regarding said copolymer, it is not clear that the copolymer what kind of behavior will acts at crosslinking procedure, and details of crosslinking is not clear. Regarding crosslinking agent which performs sulfur substitution, said synthetic polymer is hardened by crosslinking at the temperature of less than 85° C., however, since this temperature is too law, it cannot be said that a product of adequate crosslinking is obtained. Although there is no specific explanation about crosslinking, it is considered that the reaction accompanies with difficulty.
JP2008-534754 publication (Patent Document 9) is an invention relating rubber dispersion, use of the rubber dispersion for the preparation of a latex foam, method for preparation of the latex foam and the latex foam obtained by rubber dispersion of the present invention. And the invention can be illustrated as follows. That is; a reinforced latex polymer particles containing structural unit of a) 51-90 weight % of base latex polymer and b) 10-49 weight % of aromatic vinyl monomer and conjugated diene monomer, said reinforced latex polymer particles is aqueous reinforced rubber dispersion containing particles having single glass transition point (Tg) from −25° C. to 28° C. measured by differential scanning colorimeter (DSC), weight % is based on total weight of polymer particles in rubber dispersion, said base latex polymer particles has lower Tg measured by DSC than that of the reinforced latex particles. And not aiming a latex foam. Said invention is aiming to avoid disadvantage of a latex foam referring to viscoelasticity, that is, elasticity of pressed foam and recovery speed, and not aiming materials of thin rubber of the present invention.
JP2008-545814 publication (Patent Document 10) is a method to prepare an elastomer product comprising, (a) step to prepare carboxylic nitrile butadiene blended rubber composition containing 0.25-1.5 parts of zinc oxide to 100 parts of dry rubber, alkali to bring a pH over than 8.5, stabilizer, acid and at least one accelerator selected from the group consisting of guanidine, ditiocarbamate or thiazole compounds when need is arisen, (b) step to dip a former into said carboxylic nitrile butadiene blended rubber composition and (c) step to crosslink said carboxylic nitrile butadiene blended rubber composition and to form a elestomer product.
In said invention the rubber composition is crosslinked by zinc oxide. For the purpose to provide a product with desired resistance to chemicals, combination of a vulcanizing accelerator is contained. Ditiocarbamate is used as a vulcanizing accelerator. For the purpose to improve resistance to chemicals, ditiocarbamate vulcanizing accelerator and mixture of diphenyl guanidine and zinc mercaptobenzothiazole are used and better effect can be obtained. This is resulting to use sulfur and a vulcanizing accelerator for crosslinking of rubber, and a gloves prepared by said preparation method has a problem to cause IVtype hypersensitivity.
In the specification of U.S. Pat. No. 7,005,478 publication (Patent Document 11) there is a disclosure as mentioned below. That is, at the preparation of a product consisting of elastomer, react elastmer possessing a carboxyl group with (a) carboxylic acid or derivatives thereof, (b) compound containing bivalent or trivalent metal and (c) amine or amino compound in (d) condition to contain a neutralizing agent which neutralize at least a part of carboxylic acid group in said base polymer. At the reaction, a vulcanizing accelerator, thiuram and carbamate are not used. As a base polymer, natural latex rubber, synthetic latex polymer (e.g. acrylonitrile), butadiene rubber such as synthetic butadiene rubber or carboxylic butadiene rubber are mentioned as an example, however MMA is not contained. Or, does not use carboxylated acrylonitrile. In this method, said (c) amine or amino compound is an essential component. Amine group or amino group reacts with carboxylic acid derivatives and forms complex with bivalent or trivalent metal. By using complex forming reaction, stabilization becomes difficult and as the result, it is pointed out that a stabilized product is difficult to obtain.
In JP2008-512526 publication (patent Document 12), a polymer latex, wherein soft phase contains structural unit originated to at least one monomer selected from the group consisting of conjugated diene; ethylene unsaturated mono carboxylic acid, ethylene unsaturated dicarboxylic acid and anhydride thereof, monoester and mono amide; (metha)acrylonitrile, styrene, substituted styrene,α-methylstyrene, C1-C10 ester of (metha) acrylic acid, amide of (metha)acrylic acid, N-methylolamide group and esters thereof and ethylene unsaturated compound containing ether derivatives independently. Polymer latex according to any one of previously mentioned item, wherein hard phase contains structural unit originated to at least one monomer selected from the group consisting of ethylene unsaturated mono carboxylic acid, ethylene unsaturated dicarboxylic acid and anhydride thereof, monoester and mono amide; N-methylolamide group and esters thereof and ethylene unsaturated compound containing ether derivatives, mixture thereof, (metha)acrylonitrile, styrene, substituted styrene,α-methylstyrene, C1-C10 ester of (metha)acrylic acid, amide of (metha)acrylic acid and mixture thereof independently. And, crosslinking by N-methylolamide group is mentioned.
JP2010-144163 publication (Patent Document 13) discloses crosslinking by vinyl group or epoxy group.
JP2009-523164 publication (Patent Document 14) discloses combination of crosslinking bond receptor functional group and crosslinking agent.
JP2009-525411 publication (Patent Document 15) discloses self crosslinking of polyvinyl alcohol.
International publication 2005-049725 (Patent Document 16) uses organic peroxide under specific condition.
USPA2006/0057320 publication (Patent Document 17) uses alkoxy alkyl melamine and zinc oxide for crosslinking.
USPA2004/0132886 publication (Patent Document 18) uses combination of zinc oxide and peroxide as a crosslinking agent.
USPA2003/0017286 publication (Patent Document 19) uses zinc oxide, carboxylic acid, amine or amino compound as a crosslinking agent.
In Non Patent Document 1 (Andrew Kells and Bob Grobes “Cross-linking in carboxylated nitrile rubber dipped film” LATEX 24-25, January 2006 Frankfurt, Germany) reports that a latex, whose tensile strength is improved, is obtained by using tetramethylthiuram (TMTD), 2,2′-dithio-bis(benzothiazole) (MBTS), N-cyclohexylbenzothiazole-sulfinicamide (CBS), zinc diethylthio carbamate (ZDEC) and necessary additives consisting of zinc oxide with small amount of sulfur. This report shows that carboxylic nitrile latex gloves having excellent durability cannot be obtained without use sulfur or a vulcanizing accelerator based on sulfur.
A method to prepare a gloves from self crosslinking material is investigated, while, action of self crosslinking method which is necessary for preparation of desired gloves is not technically explained in the report. It is understood that sufficient results are not obtained regarding technical explanation of self crosslinking latex.
In Non Patent Document 2 (Dr. SorenBuzs “Tailored synthetic dipping lattices: New approach for thin soft and strong gloves and for accelerator-free dipping” LATEX 23-24, January 2008, Madrid, Spain) following two crosslinking methods are disclosed. That is, direct crosslinking method of NBR latex by functional reactive group (R) instead of conventional sulfur crosslinking, and crosslinking method of NBR latex by ionic bond formed between carboxyl group of NBR latex by zinc oxide. And, the document reports that this method is promising as a future technique. Unfortunately, there is no specific explanation about functional group which acts in covalent bond. And is not succeeded to make clear the concrete method for forming.
In Nichias technical information (Vol 5, 2000, No. 321), combination of epoxidated natural rubber and carboxylic NBR is used for self crosslinking.